As the exposure pattern is miniaturized to comply with the increasing integration density of VLSIs, the lithography apparatus or steppers for printing circuit patterns on semiconductor wafers use light sources of shorter wavelengths. The exposure tool light source changed from the prior art i-line (wavelength 365 nm) to KrF excimer laser (248 nm) and recently to ArF excimer laser (193 nm).
As the light source becomes of shorter wavelength, not only lenses and optical members used in the exposure tool, but also synthetic quartz glass substrates for photomasks serving as the original of IC circuit are required to be of higher precision.
Particularly in the ArF excimer laser application, the suppression and uniformity of initial absorption of laser irradiation, the presence of residual birefringence in glass substrates, and a change of birefringence during laser irradiation are of significance. The birefringence in synthetic quartz glass is attributable to the stress remaining in the material. An appropriate annealing treatment for stress removal is effective for reducing birefringence.
For example, Patent Document 1 discloses a method involving heat treating synthetic quartz glass having a hydroxyl (OH) concentration of 230 ppm at 1,100° C. for 200 hours, and annealing to 500° C. at −20° C./hr, for thereby reducing the birefringence to 10 nm/cm or less. Patent Document 2 discloses a method involving heat treating synthetic quartz glass having a hydroxyl concentration of 800 to 1,300 ppm at 1,000° C. for 10 hours, and annealing to 500° C. at −10° C./hr, for thereby reducing the birefringence to 2.0 nm/cm or less. Patent Document 3 discloses a method involving heating a synthetic quartz glass block at a first holding temperature in the range of at least 900° C., holding at the temperature for a certain time, cooling down to a temperature of 500° C. or below at a cooling rate of up to 10° C./hr, heating at a second holding temperature in the range of 500 to 1,100° C., holding at the temperature for a certain time, and cooling down to a temperature of 100° C. lower than the second holding temperature at a cooling rate of at least 50° C./hr, for thereby improving the inhomogeneity of birefringence value distribution.
The method of Patent Document 1, however, requires a heat treatment time in excess of 200 hours. A long time of annealing is undesirable from the aspects of a productivity drop and contamination with impurities from the treating environment.
In the method of Patent Document 2, the difference (ΔOH) between maximum and minimum hydroxyl concentrations is limited to 50 ppm or less. There is a problem that if ΔOH is outside the range, the birefringence becomes more than 2 nm/cm. In addition, since the step of holding at 1,000° C. for a certain time must be followed by the step of annealing to 500° C. at a rate of −10° C./hr, the overall heat treatment takes a long time.
The method of Patent Document 3 involves holding at a first holding temperature for a certain time, cooling down below a second holding temperature, and again heating at the second holding temperature. Thus a long time is also necessary for heat treatment. There is a need for a method of producing synthetic quartz glass capable of reducing birefringence through brief annealing.